One step in the manufacture of semiconductor integrated circuits is the bonding of a silicon chip or die with an adhesive to a copper frame from which extend metal conductor leads. The bonded die and lead frame assembly is encapsulated within a polymeric sealant and connected to external circuitry by way of the metal conductor leads that extend through the encapsulation.
Epoxy compounds are preferred as the die attach or encapsulating adhesive due to their superior adhesive strength. The epoxies conventionally used are the aromatic epoxies, due to their strength, but these are inherently rigid and brittle. During the manufacturing process the adhesives and substrates are subjected to repeated thermal cycling. If the adhesives and substrates have widely disparate coefficients of thermal expansion, the stress of thermal cycling can lead to adhesive failure, substrate warpage, or fracture of the die. Thus, a crucial requirement for an adhesive destined for microelectronics use is that it be strong and flexible to absorb the stress of thermal cycling.
A second crucial criterion is that the adhesives be capable of rapidly curing to meet the speeds of assembly line processing. The fast cure times required, typically 30-60 seconds at about 200.degree. C., are known as snap-cure. This combination of criteria, adhesive strength, flexibility, and ability to be snap-cured, is difficult to attain in one adhesive.
It is also crucial that the epoxy formulations be free of ionic contamination, particularly sodium and chloride ions, and free of bonded chlorine. These contaminants can cause corrosion of the metal leads in semiconductor devices and the ultimate failure of the devices.
To add flexibility, epoxies can be co-reacted with an aliphatic flexibilizer; this, however, reduces adhesive strength because the level of aromatic moieties is lowered. The addition of a flexibilizer also reduces snap-cure because the flexibilizer has a high molecular weight per epoxy. Moreover, when the aliphatic and aromatic epoxies are cured, they may not co-react due to a difference in reactivity rates; low molecular weight compounds may volatilize out before cure, and high molecular weight compounds may not completely cure. This combination of factors, sometimes even a problem for slow cure formulations, is fatal for achieving snap-cure.
As a possible solution to some of these problems, it is known to combine aromatic moieties with aliphatic moieties in the backbone of the same polymer, but the currently available polymers of this type have a high ratio of aromatic to aliphatic moieties, which results in a loss of flexibility. Additionally, the method of preparation of these materials results in high chlorine contamination, which is deleterious in microelectronics applications. The polymers also have high viscosities, which requires the addition of a solvent as a diluent. During snap-cure, the curing is sometimes faster than complete solvent volatilization, which leads to voids in the cured adhesive, and potential failure of the microelectronics chip or device. Thus, low viscosity materials, which eliminate the need for solvent, are preferred.
These problems make formulation of snap-cure adhesives difficult and create a continuing need for snap-curable adhesives that have strength and flexibility.